1. Field of the Invention
The present invention concerns a method of measuring altimetric parameters and a device for implementing the method. The method and the device are typically intended to be used on a satellite.
2. Description of the Prior Art
As shown in FIG. 1, a prior art device for implementing a measuring method of this kind transmits a signal SE vertically which is reflected by the sea M or the ice as a reflected echo SR. By appropriately processing this reflected echo signal SR it is possible to calculate parameters such as the altitude of the satellite, the speed of the wind over the surface of the sea, the height of the waves, etc. In addition to the precise altitude of the satellite, this information offers an assessment of the dynamics of oceans or of the condition of seas.
Referring to FIG. 2, in one embodiment a measuring device of the type for implementing the invention includes a sequencer 10, a ramp signal generator 11, a first amplifier 12, a circulator 100, an antenna 13, a second amplifier 14, a mixer 15, an automatic gain control unit 16, a time-delay unit 16', a demodulator 17, a fast Fourier frequency transform unit 18, a tracking unit 19 and an estimator unit 20.
In a first prior art technique for measuring altitude (to the nearest centimeter), very short pulses are transmitted, having a duration in the order of 0.1 ns, and an altitude is measured as a function of a round trip time for the pulses between the satellite and the ground. Although this technique can be used in the context of the invention, it has two drawbacks. Firstly, it requires a high power transmitter and, secondly, it is characterized by a low signal to noise ratio for the reflected echo signal SR. The use of a long pulse with linear frequency modulation to remedy this is known in itself. To this end, the sequencer 10 produces control pulses I applied to an input of the ramp generator 11. In response, the latter periodically produces long pulses with linear frequency modulation. For example, a pulse I' produced by the generator 11 sweeps a 330 MHz band of frequencies about a center frequency of 900 MHz. After amplification by the amplifier 12 the frequency modulated pulses I' are transmitted by the antenna 13 as the transmitted signal SE.
After reflection from the sea, the transmitted signal SE is received by the antenna 13 as the reflected echo signal SR. This signal SR has characteristics close to those of the transmitted signal SE, although subject to attenuation and to a time-delay equal to the time required for the transmitted signal to make the return trip between the satellite and the Earth.
The pulses Ir forming the reflected echo signal SR are mixed with reference pulses Iref in the mixer 15. The pulses Ir and Iref are both subject to linear frequency modulation and any time-shift between the pulses Ir and Iref is reflected in a frequency value at the output of the mixer 15. Accordingly, a pure frequency is ideally obtained at the output of the mixer 15 for a point target. In reality, and in particular because of the presence of waves V on the surface of the sea M, as will be described below with reference to FIG. 4, the spectrum of the signal obtained at the output of the mixer 15 is spread over a relatively wide band of frequencies. Analysis of this spectrum provides all of the information that the measuring device is required to yield.
The advantage of this so-called "full deramp" mixing technique is that a time difference is converted into a frequency variation. The aim is to eliminate the effects of all transit times in the amplification and processing stages. However, it has the disadvantage of making the measuring device synchronous, as a result of defining an extremely narrow working window, which requires fine tracking using a control loop including the tracking unit 19, as will be described below.
The signal from the mixer 15 is passed through the automatic main control unit 16 to an input of the demodulator 17 which carries out coherent demodulation and the two quadrature output channels of which drive the K-point fast Fourier transform unit 18, where K is an integer that is a power of 2. At the output of the unit 18 there are produced series each of K samples Vi (k) where i is the number of the series and k is the number of a sample within a given series.
As shown in FIG. 3, the successive samples of a given series Vi (k) define a spectrum of particular shape resulting from the reflection of the transmitted signal SE from the sea surface. FIG. 4 shows the modeled representation of this signal, as discussed in the article "The average Impulse Response of a Rough Surface and Its Applications" by Gary S. BROWN published in Transactions on antennas and propagation, vol. AP-25, No 1, January 1977, pages 67-74. The spectral signal represented in analytical form by way of simplification comprises a first zone Z1 where the amplitude is at a low level corresponding to a thermal noise level N, a second zone Z2 with a sharply increasing slope H ending at a peak, and a third zone Z3 with a decreasing slope .xi.. It can be shown that the slope H of the signal in the second zone Z2 defines the height of the waves on the surface of the sea and that the slope .xi. of the signal in the third zone is associated with depointing of the antenna 13 of the measuring device, the level difference .sigma. between the level of the spectral signal in the zone Z1 and the peak defines a back-scattering coefficient associated with the wind speed, and the projection of the middle point P of the second zone onto the abscissa axis k is associated with a round trip time .tau. between the satellite and the sea.
As previously indicated, the full deramp technique has the drawback of rendering the measuring device synchronous, as the result of defining an extremely narrow working window corresponding to the K points of the fast Fourier transform (FFT). This then requires fine tracking to center the spectral signal obtained in the working window (see FIG. 4). The control loop including the tracking unit 19 fulfills this function. It guarantees that the zone Z2 of the spectral signal with the suddenly increasing slope is positioned at given points of the fast Fourier transform and that the maximal amplitude of the spectral signal is maintained at a given level.
To this end, the tracking unit 19 receives the successive series of K samples Vi (k) and produces first and second control signals Ci and C'i respectively applied to a control input of the automatic gain control unit 16 and to a control input of the time-delay unit 16'. The time-delay unit 16' is adapted to delay each pulse received from the ramp signal generator 11, in accordance with the received control signal C'i, so producing a reference pulse Iref applied to the second input of the mixer the first input of which receives the pulses Ir forming the reflected echo signal SR.
The level of the control signal Ci for iteration i if calculated according to the level of the control signal for iteration (i-1) and an error signal (.epsilon.Ci) obtained from the series of samples Vi-1 (k). To calculate this error signal (.epsilon.Ci), the energy is calculated in sliding windows of eight successive samples shifted in steps of 1 sample each time. The energy value measurements are then obtained: ##EQU1## j varying between 1 and (K-8)
The maximal value Emax from all of these energy values {Ej} is then defined. The error signal (.epsilon.Ci) is then defined by: EQU (.epsilon.Ci)=10 Log (Emax/8.Vnom)
where Vnom is the required nominal value of the peak level for the spectral signal shown in FIG. 4. In this way the spectral signal is maintained at a given level.
By means of appropriate processing that will be evident to the person skilled in the art, the level of the control signal C'i for iteration i is calculated according to the level of the control signal for iteration (i-1) and an error signal (.epsilon.C'i) obtained from the series of samples Vi-1 (k). This control signal C'i is produced in order to center the spectral signal shown in FIG. 4 in the window formed by the k points of the fast Fourier transform, in particular so that the middle point of the zone Z2 with suddenly increasing slope is at all times positioned on the same point k1 of the fast Fourier transform.
Because of this tracking, the spectral signal obtained from the fast Fourier transform unit is then at a constant position within the window defined by the points of the FFT. More detailed information on the use of these tracking operations are given in a thesis submitted Sep. 27, 1985 by Jean-Paul DUMONT to the Institut National Polytechnique de Toulouse under the title "Estimation optimale des parametres altimetriques des signaux radar POSEIDON" ("Optimal estimation of altimetric parameters of POSEIDON radar signals").
Maintaining the spectral signal within a window offers the possibility of processing said signal to measure information characterizing the surface of the Earth to be studied.
This information characterizing the surface of the Earth is obtained using the estimator unit 20 which is typically in the form of processor means implementing the so-called Maximum Likelihood Estimator (MLE) algorithm.
In the prior art, the values of:
(a)--the slope H of the signal in the second zone Z2 defines the height of the waves at the surface of the sea, PA1 (b)--the level difference .sigma. between the level of the spectral signal in the zone Z1 and the peak level defining the back-scattering coefficient associated with the wind speed, and PA1 (c)--the projection of the middle point of the second zone onto the abscissa axis k, associated with a round trip time .tau. between the satellite and the sea, are calculated using the MLE algorithm applied to all of the curve defined by the samples of a series Vi (k) (FIG. 4). PA1 (1)--the response of the sea to a Dirac pulse, PA1 (2)--the response of the measuring device for a point target, and PA1 (3)--the distribution of the height of the null slope points of the surface. PA1 transmitting measurement pulses towards the surface of the sea, PA1 frequency transformation of a resultant signal resulting from the reflection of said pulses at the surface of the sea into a spectral signal of samples successively comprising: PA1 (a)--a first zone with a low amplitude level corresponding to a thermal noise level, PA1 (b)--a second zone with a sharply increasing slope ending at a peak and dependent on the height of the waves at the surface of the sea, a back-scattering coefficient and the altitude of said satellite, and PA1 (c)--a third zone of decreasing slope associated with depointing of an antenna of the measuring device, and PA1 estimating from said samples of said spectral signal, by means of maximum likelihood processing using a model ignoring depointing of the antenna, at least one of the following values: the height of the waves at the surface of the sea, the back-scattering coefficient and the altitude of the satellite, PA1 in which method samples of said spectral signal are selected within a selection zone that corresponds to said first and second zones for a predetermined maximal level of the height of the waves at the surface of the sea, said maximum likelihood processing being applied only to the selected samples. PA1 means for transmitting measurement pulses towards the surface of the sea, PA1 means for frequency transformation of a resultant signal resulting from the reflection of said pulses at the surface of the sea to produce a spectral signal of samples successively comprising: PA1 (a)--a low amplitude level first zone corresponding to a thermal noise level, PA1 (b)--a second zone with a sharply increasing slope ending at a peak and dependent on the height of the waves at the surface of the sea, a back-scattering coefficient and the altitude of said satellite, and PA1 (c)--a decreasing slope third zone associated with depointing of an antenna of said measuring device and, PA1 means for estimating from said samples of said spectral signal, by means of maximum likelihood processing using a model ignoring depointing of said antenna, at least one of the following values: the height of the waves at the surface of the sea, the back-skattering coefficient and the altitude of the satellite, the device further including means for selecting samples of said spectral signal in a selection zone that corresponds to said first and second zones for a predetermined maximal level of the height of the waves at the surface of the sea and said maximum likelihood processing being applied only to these selected samples.
To this end, in a manner that is conventional in MLE analysis, the curve of the spectral signal from FIG. 4 is approximated by a model. In the present context, the model is the Brown model which specifies that the spectral signal is the result of the convolution of three terms, namely:
For more information on this model, reference may usefully be had to the article previously mentioned "The average Impulse Response of a Rough Surface and Its Applications" by Gary S. BROWN published in Transactions on antennas and propagation, vol. AP-25, No 1, January 1977, pages 67-74.
In the Brown model, the curve is modeled in the following manner, where V (t) is the time equivalent of the spectral component Vi (k): EQU V(t)=.sigma./2.{1+erf(t-.tau.)/(.sqroot.2..sigma..sub.c)!}, for t.ltoreq..sigma.; and EQU V(t)=.sigma./2.{1+erf(t-.tau.)/(.sqroot.2..sigma..sub.c)!}.exp-.alpha.(t- .tau.)!, for t.gtoreq..sigma.
where .sigma. and .tau. are the parameters introduced in discussing FIG. 4, EQU .sigma..sub.c =.sqroot.0.513.T+(H/2c)!
where T is the duration of the frequency modulated pulses, H is the wave height parameter and c is the speed of light, and EQU .alpha.=(4.c/.psi..H),
.psi. being equal to 0.725.sin.sup.2 (.theta.) and .theta. being the aperture angle of the antenna 13 of the measuring device.
By defining a model of this kind and using an MLE algorithm that is well known to the person skilled in the art, the prior art approximates the respective values assumed by the parameters H, .sigma. and .tau. for the curve of the spectral signal Vi (k) concerned for values of k between 1 and K. This MLE analysis is applied to all of the samples of a series Vi (k), possibly previously averaged to reduce speckle. This solution has the main disadvantage of leading to the following compromise.
Either the number of samples in the zone Z3 is reduced to a minimal number so that the calculated values assumed by the parameters H, .sigma. and .tau. are not skewed (a result of this is a somewhat imprecise measurement of the value of the slope parameter .xi.) or the number of samples in the zone Z3 is increased to a higher number to obtain a more precise measurement of this value of the slope parameter .xi.. This skews the measurement of the values assumed by the parameters H, .sigma. and .tau. in the event of depointing the antenna and the prior art in the form of the thesis submitted Sept. 27, 1985 by Jean-Paul DUMONT to the Institut National Polytechnique de Toulouse under the title "Estimation optimale des parametres altimetriques des signaux radar POSEIDON" ("Optimal estimation of altimetric parameters of POSEIDON radar signal"), pp. 215-216, suggests a posteriori correction of the measurement of the values assumed by the parameters H, .sigma. and .tau. according to the value assumed by the parameter .xi., which is calculated separately. This correction nevertheless results in imprecise calculations of the value of these parameters.
The invention is aimed at overcoming this disadvantage, notably by providing a measuring device adapted to provide precise measurements of the values assumed by all of the parameters H, .sigma., .tau. and .xi..